1. Field of the Invention
The present invention relates generally to an apparatus for controlling the time base of a video signal and, more particularly, is applicable to the control of the time base of a video signal recorded intermittently by a video tape recorder in performing surveillance or the like.
2. Description of the Prior Art
Video tape recorders are known which are suitable for making intermittent recordings, for example, in conducting surveillance for security purposes. In its recording mode, this video tape recorder (VTR) serves to record a single frame of a video image at a given time while the magnetic recording tape in the recorder is held stationary. Once this is accomplished, the tape is advanced to a new position where it is likewise held stationary for a predetermined time interval after which a single frame of a subsequent image is recorded. This intermittent recording process enables the VTR to record events over a substantially longer time period than a similar recorder operating in a continuous recording mode.
Japanese Laid Open Patent Gazette No. 63-133,339 describes such an intermittent recording VTR which is operative, in its reproducing mode, to transport a magnetic recording tape continuously to review images that have been recorded thereon. Once a desired picture is located in the course of such review, it can be observed more fully by carrying out a so-called frame playback of the picture. In the use of such intermittent recording VTR, the pictures reproduced in the continuous playback mode should be as free of noise as possible. In addition, the intermittent recording VTR should be compatible with standard video tape recorders and adjustments required in the course of manufacturing the intermittent recording VTR should be easy to make.
Accordingly, a VTR to meet these objectives is set forth in Japanese Patent Application No. 62-204,498, which has been assigned to the assignee of the instant application. This VTR employs a pair of rotary magnetic heads having differing azimuth angles and which are spaced apart in the direction of the axis of rotation by a predetermined step difference. In its recording mode, such intermittent recording VTR is operative to record a single video frame on a stationary magnetic tape, together with a plurality of pilot signals to assist tracking in the playback mode. In continuous playback, the pilot signals assist each head to scan a corresponding track to reproduce a field of the recorded image which is then stored in memory. This permits the VTR to carry out suitable signal processing to compensate for drop-outs of the reproduced signal resulting from the failure of the head to scan the corresponding track precisely. As thus processed, each field reproduced by one head is read-out from the memory as the next succeeding field is being scanned by the other head. In this manner, a continuously reproduced picture is provided having substantially reduced noise especially near the upper and lower extremes of the picture where tracking is normally poorest in the continuous playback mode.
In order to assist in locating any particular recorded images which it is desired to review, respective alarm numbers, together with the recording date and/or time, are assigned to each image and data signals representing this information are recorded therewith. When each image is reproduced subsequently, the data signals are utilized by a computer to control the production by a character generator of corresponding character images representing the alarm number and date and/or time. These character images are then displayed on a monitor screen together with the respective recorded picture. In the exemplary illustration of FIG. 1, a number display area RN and a date and/or time display area RT are defined on a monitor screen P, wherein the alarm number and the date and/or time of the observed image are displayed on every reproduced picture with the use of a character generator controlled by a computer.
The time base of a video signal reproduced by a video tape recorder fluctuates, so that the reproduced video signal exhibits unavoidable and undesirable movement or jitter due to a jitter signal component of the video signal resulting from such fluctuations. However, the character generator is arranged to produce video character signals according to the standards of the original video signal, so that these video character signals do not possess the jitter signal component of the reproduced video signal.
Accordingly, when the reproduced video signal and the video character signals are combined without signal processing to remove the undesirable consequences of the jitter signal component in the reproduced video signal, the character displayed as a result of the video character signal (FIG. 2) appears to exhibit distortion in the horizontal direction of the picture by reason of its contrast with the reproduced video signal having a jitter signal component and/or the reproduced video signal exhibits increased or more apparent jitter by reason of its contrast with the displayed character which is generated without a jitter signal component.
Although the jitter signal component of the reproduced video signal can be minimized or removed with the use of a time base corrector, the reproduced video signal includes a large amount of information. Consequently, such a time base corrector requires large scale circuitry which is undesirably complicated and expensive.